Subcritical reactor

The nuclear weapon material plutonium-239 is also suitable although it can be expended in a cheaper way as MOX fuel or inside existing fast reactors.

However, even the issue of decay heat can be minimized as a subcritical reactor needn't assemble a critical mass of fissile material and can thus be built (nearly) arbitrarily small and thus reduce the required thermal mass of an emergency coolant system capable of absorbing all heat generated in the hours to days after a scram.

Another issue in which a subcritical reactor is different from a "normal" nuclear reactor (no matter whether it operates with fast or thermal neutrons) is that all "normal" nuclear power plants rely on delayed neutrons to maintain safe operating conditions.

By contrast this means that too low a fraction of delayed neutrons makes an otherwise fissile material unsuitable for operating a "conventional" nuclear power plant.

Conversely, a subcritical reactor actually has slightly improved properties with a fuel with low delayed neutron fractions.

It just so happens that while 235U the currently most used fissile material has a relatively high delayed neutron fraction, 239Pu has a much lower one, which - in addition to other physical and chemical properties - limits the possible plutonium content in "normal" reactor fuel.

There are technical difficulties to overcome before ADS can become economical and eventually be integrated into future nuclear waste management.

There are concerns about the window separating the protons from the spallation target, which is expected to be exposed to stress under extreme conditions.

Carlo Rubbia, a nuclear physicist, Nobel laureate, and former director of CERN, was one of the first to conceive a design of a subcritical reactor, the so-called "energy amplifier".

However, the costs for construction, safety and maintenance of such complex installations are expected to be very high, not to mention the amount of research needed to develop a practical design (see above).

There exist cheaper and reasonably safe waste management concepts, such as the transmutation in fast-neutron reactors.

For future waste management, a few transmutation devices could be integrated into a large-scale nuclear program, hopefully increasing only slightly the overall costs.

The main challenge facing partitioning and transmutation operations is the need to enter nuclear cycles of extremely long duration: about 200 years.

[3] Another disadvantage is the generation of high quantities of intermediate-level long-lived radioactive waste (ILW) which will also require deep geological disposal to be safely managed.

Both positive and negative aspects were examined in an international benchmark study[4] coordinated by Forschungszentrum Jülich and financed by the European Union.

[citation needed] One such proposal calls for a gas-cooled fast reactor that is fueled primarily by plutonium and americium.